Conventionally this type of positioner is designed so as to operate with an electric current between 4 and 20 mA sent through a pair of electric wires from a higher-level system. For example, if a current of 4 mA is sent from the higher-level system, the opening of the regulator valve is set to 0%, and if a current of 20 mA is sent, then the opening of the regulator valve is set to 100%.
In this case, the supplied electric current from the higher-level system varies in the range of 4 mA (the lower limit electric current value) through 20 mA (the higher limit electric current value), and thus the internal circuitry within the positioner operates on an electric current of no more than the 4 mA that can always be secured as an electric current value that is supplied from the higher-level system (See, For Example, Japanese Unexamined Patent Application Publication 2004-151941). Note that this electric current is known as the minimum operating electric current, and is, for example, about 3.8 mA.
FIG. 7 is illustrates the critical components in a conventional positioner. This positioner 100 receives a supply of an electric current I through a pair of electric wires L1 and the L2 from the higher-level system 200 and produces its own operating power supply from the electric current I that is supplied, and, on the other hand, also controls the degree of opening of a regulator valve, not shown, in accordance with the value of the supplied electric current I.
The positioner 100 is provided with a main circuit 2 that includes a CPU (a central calculation processing portion) 1, constant voltage circuit 3 that includes a zener diode D1, and electro-pneumatic converting portion 4 for converting an electric signal from the main circuit 2 into an air pressure signal, and a resistor R1.
The constant voltage circuit 3 and the resistor R1 are connected in series between the terminals T1 and T2 through which the current I from the higher-level system 200 is inputted/outputted, and the connecting point between the constant voltage circuit 3 and the resistor R1 is grounded. Note that the electro-pneumatic converting portion 4 has a coil 4-1 for driving a nozzle flapper mechanism (not shown), where the electric signal from the main circuit 2 is supplied to the coil 4-1.
In this positioner 100, the constant voltage circuit 3 produces a constant voltage V1 from the supply electric current I from the higher-level system 200, and supplies that produced constant voltage V1 to the main circuit 2 as an operating power supply. The CPU 1 in the main circuit 2 inputs a voltage Vs that is produced on the outflow side of the electric current I of the resistor R1 as the target opening θ sp, that is, inputs, as the target opening θ sp, a voltage Vs in accordance with the value of the supply electric current I from the higher-level system 200, and inputs a voltage Vr that is fed back from the regulator valve as the actual openings θ pv of the regulator valve, and supplies an electric signal to the coil 4-1 of the electro-pneumatic converting portion 4 such that the target opening θ sp and the actual opening θ pv match.
In this circuit structure, an electric current I that varies between 4 and 20 mA is supplied from the higher-level system 200, and of this supply electric current I, an electric current that is consumed by the circuit (the operating electric current) Ic flows to the line LA on the main circuit 2 side, where any surplus electric current above this operating electric current Ic flows to the line LB on the constant voltage circuit 3 side as a surplus electric current Iv (where Iv=I−Ic). Moreover, as illustrated in FIG. 8, an electric current Id of between 0 and 0.7 mA flows in the coil 4-1 of the electro-pneumatic converting portion 4 depending on the change in the 4 through 20 mA (θ sp=0 through 100%) in the supply electric current I.
On the other hand, in the positioner disclosed in Japanese Unexamined Patent Application Publication H11-304033 (Japanese Patent Number 3596293) (“JP '033”) and Japanese Unexamined Patent Application Publication 2000-304148 (Japanese Patent Number 3635982) (“JP '148”), the coil for the electro-pneumatic converting portion and the other circuits are connected in series, and the coil for the electro-pneumatic converting portion is provided on the upstream side. Doing so makes it possible to increase the electric current that flows in the coil of the electro-pneumatic converting portion, increasing the margin of stability relative to noises such as temperature, and the like, and increasing the responsiveness of the regulator valve.
However, while even though the electric current that flows in the electro-pneumatic converting portion is larger in the methods disclosed in JP '033 and JP '148, the internal circuitry is connected in series (where the coil of the electro-pneumatic converting portion and the other circuits are connected in series), and thus the impedance within the internal circuitry is high, increasing the terminal-to-terminal voltage for the positioner to operate (the minimum operating terminal voltage). Because of this, it is difficult to connect two positioners in series (double connection) between two transmission lines (a pair of electric wires), and difficult to achieve methods of use wherein other loads are connected.
For example, when the supply voltage from the two transmission lines is 15 V, then if two positioners were connected in series, then the terminal voltages of the positioners would be 7.5 V. In this case, because the internal circuitry is connected in series in the methods set forth in JP '033 and JP '148, the minimum operating terminal voltage of the positioner is 12 V, making it impossible to connect two positioners in series between two transmission lines. In this case, a supply voltage of 24 V from the two transmission lines is necessary in order to connect two positioners in series.
Moreover, in recent years there have been requests for incorporating, as added functions, functions such as regulator valve fault diagnostic and fault self-diagnostic, and the like, in addition to the primary function (the basic function) of controlling the degree of opening of the regulator valve in a positioner. However, when responding to such demands for multifunctionality, it is necessary to allocate electric current for operating the function circuit portions for the added functions, where these portions necessarily reduce the electric current allocated to the function circuit portion for the basic function, which may compromise the basic function.
The examples of the present invention were created in order to solve this type of problem, and the object thereof is to provide a positioner able to keep the impedance of the internal circuitry low and to operate the function circuit portions of added functions, such as fault diagnostic of the regulator valve and fault self-diagnostic, without increasing the electric current to the function circuit portions such as the electro-pneumatic converting portion, and without sacrificing the basic function.